Membranes have been employed which use the principle of selective permeation to separate mixtures of gases into their components. Each gas has a characteristic permeation rate that is a function of its ability to dissolve into and diffuse into a given membrane or molecular sieve. Typically, membranes are produced as either flat sheets or as hollow fibers. Flat sheets are usually employed in spiral wound separators while hollow fibers are employed in shell and tube type separators. Other innovative configurations employing flat sheet or hollow fiber membranes are also available.
In a bundle of hollow fiber membranes positioned by various means and orientations within an elongated shell, one or more gases are separated from a mixture of gases by allowing the gases to permeate selectively through the membrane. For example, relatively pure nitrogen can be made from air by feeding compressed air into one end of an elongated container filled with a plurality of juxtaposed axially hollow fiber membranes running longitudinally of the container. The feed can enter the bores of the fibers or the outside of the fibers. Oxygen, carbon dioxide, water and other gases will permeate through the membrane fibers, but nitrogen permeates at a much slower rate. The gases passing through the membrane and separated from the mixture of gases are withdrawn from the low pressure side of the membrane. The portion of the air which does not permeate the membrane fibers after contact with the active membrane surface is relatively pure nitrogen. Generally, the nitrogen gas will have a small amount of oxygen. Most mixtures of gases, such as air, contain some water vapor. The amount of water vapor in the gas is typically characterized by the dew point temperature of the gas mixture. The dew point is the temperature at which water will begin condensing from the gas mixture at its pressure. Water vapor successfully passes through the membrane fibers but condensed water has a tendency to clog the pores of the membrane fibers. For this reason, h is highly desirable to operate the membrane at a temperature which is a few degrees above the dew point of the feed.
Conventional membrane separation of air into its components involves compressing air, then passing the air through an aftercooler to remove the heat of compression. Water or air cooled aftercoolers generally cool the gas to about 35.degree. C. or higher, and at or near its dew point. If the compressed gas is cooled below its dew point in the aftercooler, water vapor will condense from the gas. These condensates are removed before further processing.
To minimize moisture condensation in the membrane, one approach has been to preheat the feed to the membrane system to above its dewpoint, and then to insulate the individual membrane modules, thereby retaining the heat in the modules. Thus, the membrane operates above the dewpoint of the feed stream and condensation in the membrane is avoided. Heating of the feed gas is typically effected from external sources such as electrical heaters. Alternatively, the membrane modules themselves can be enclosed in an insulated heated container which keeps the membrane modules at a temperature above the dewpoint of the feed gas. Thus, saturated feed air at a lower temperature may be fed to the membrane modules without condensation of the moisture on the membrane surface. While both approaches and combinations thereof, avoid condensation in the membrane, the membranes operate at elevated temperatures. This introduces an inefficiency in the separation process since the ability of a membrane to separate gases typically increases as the temperature is lowered.
In attempts to predry the feed stream to permit the membrane to be operated at lower temperatures, refrigerated dryers and adsorbent beds have been used to lower the dewpoint of the feed gas below the desired operating temperature of the membrane unit. A refrigerated dryer provides a cooled, low dew point gas stream to feed to the membrane unit; however, the capital and operating costs associated with a refrigerated dryer are quite high, and, also, the refrigerated dryer involves the use of environmentally undesirable compounds such as freon refrigerants. When an adsorbent bed is used to remove water, the gas is not cooled, and, thus, the problems of high feed temperature prevail. In addition, some means of regenerating the adsorbent bed material must be provided.
In a typical membrane-based air separation system, a number of individual membrane modules or separators may be employed. In addition, these separators may be arranged to operate in parallel, in series, or a combination thereof. When a series arrangement is employed, a variety of recycle schemes can be employed to improve the overall efficiency of the system. For example, when air is the feed gas, the optimum configuration is dependent on the desired purity of the enriched nitrogen and/or oxygen streams, flow rates, and economic considerations.
Adsorption-based air separation systems typically employ carbon molecular sieves or zeolftes. Systems using carbon molecular sieves make use of the fact that oxygen molecules diffuse into the material much faster than nitrogen. Systems using zeolites rely on the fact that these materials have a much greater affinity for nitrogen than for oxygen. Cycles and operating conditions can be selected for either system to provide for the efficient production of high purity oxygen and/or nitrogen. Cyclic systems which operate between high pressure and atmospheric pressure (as in nitrogen PSA generators) or sub-atmospheric pressure (as in oxygen VSA generators) may be employed to provide the driving force for the separation. In either case (PSA or VSA), the operation is referred to as pressure swing adsorption (PSA) hereinafter.
Because the efficiencies of carbon molecular sieves and zeolite adsorbents to separate air are reduced by any moisture present in the feed air, prebeds containing adsorbents which reversibly bind water and/or refrigerated dryers, are used to reduce or eliminate the moisture contained in the feed gas. The refrigerated dryer is generally used where the optimum or preferred feed gas temperature is below that which can be achieved by heat exchange with available cooling tower water or ambient air. High capital and operating costs associated with refrigerated dryers plus the fact that they use environmentally undesirable compounds, such as freon refrigerants, however, limit their use in these applications.
The present invention provides a process for cooling the feed gas to a suitable temperature to optimize the performance of gas separation systems. The process is free of the limitations present in refrigerated dryers and adsorption drying processes.